1. Field of the Invention
The present invention generally relates to a developing device, a process cartridge using the developing device, and an image forming apparatus using the process cartridge, which developing device provides a toner carrier capturing roller to capture toner carriers adhered onto an image carrier. The image forming apparatus is a copying machine, a printer, a facsimile machine, and a multifunctional peripheral providing the above functions, which image forming apparatus uses an electrophotographic system.
2. Description of the Related Art
Conventionally, in an image forming apparatus such as a copying machine and a printer, a toner carrier capturing roller (toner carrier capturing member) for magnetically capturing toner carriers adhered onto an image carrier such as a photosensitive drum has been provided in a developing device which contains a two-component developer formed of toners and toner carriers (refer to Patent Documents 1 through 3). The two-component developer may include an additive.
In detail, the developing device provides a developing roller (developer carrier) disposed to face the image carrier such as the photosensitive drum for carrying the developer. The developing roller provides a sleeve which rotates in a predetermined direction and a magnet fixed in the sleeve. A magnetic field is generated to form particle clusters (grain clusters of the developer) on the circumference surface of the developing roller (sleeve), and an image developing process is executed while the developer is carried in the sleeve rotating direction.
The toner carrier capturing roller is disposed at the downstream side of the developing roller in the developing roller rotating direction while facing the image carrier. The toner carrier capturing roller provides a sleeve which rotates in a predetermined direction and a magnet fixed in the sleeve. A magnetic field is generated on the circumference surface of the toner carrier capturing roller (sleeve) so as to capture the toner carriers adhered onto the image carrier. The toner carriers captured on the toner carrier capturing roller are mechanically scraped by a scraper which contacts the toner carrier capturing roller, and the scraped toner carriers are returned to the developing device.
The developing device prevents forming an abnormal image such as a whitened image and a lighting-bug like image on the image carrier caused by the adhesion of the toner carriers on the image carrier.    [Patent Document 1] Japanese Laid-Open Patent Application No. 6-11970    [Patent Document 2] Japanese Laid-Open Patent Application No. 9-6139    [Patent Document 3] Japanese Laid-Open Patent Application No. 6-230668
The developing device can capture the toner carriers adhered onto the image carrier; however, the load for the scraper which scrapes the toner carriers captured on the toner carrier capturing roller becomes large. Specifically, when the amount of the toner carriers captured by the toner carrier capturing roller becomes large, some toner carriers may not be scraped by the scraper, and in addition, the scraper and the toner carrier capturing roller may be worn out. In this case, an abnormal image may be formed.
Especially, in a high-speed image forming apparatus (having a high processing line speed), toner carriers carried by the developing roller are likely to be dropped from the surface of the developing roller, and the amount of the toner carriers to be adhered onto the image carrier is likely to be large.